We present a mathematical model to describe the ultrafiltration behavi
our of polymer-electrolyte mixtures. The model combines the proper the
rmodynamic forces (pressure, chemical potential and electrical potenti
al differences) with multicomponent diffusion theory. The model is ver
ified with experimental data on the ultrafiltration of aqueous solutio
ns of PEG-4000 and potassium phosphate. The single solute rejection of
PEG-4000 goes through a maximum as also found by others. The single s
olute rejection of potassium phosphate depends on the ionic strength o
f the solution. At low ionic strength rejections are found of 50%. Sol
utions containing a high concentration of PEG-4000 and potassium phosp
hate show a negative rejection for potassium phosphate. This is caused
by the strongly non-ideal behaviour of these aqueous solutions. The m
odel predicts the behaviour of single solute experiments quite well, b
ut some deviations are found with the mixed solute experiments. Howeve
r, negative rejections found in the mixed solute experiments are predi
cted by the model.